There have been many cases at a construction site, building site, etc. where the earth should be deeply excavated to form a hole having a depth which is too long for its diameter. For example, there have been cases for excavating the earth to form a hole in which an anchor supporting a steel tower is embedded, a hole in which a water purifier tank is embedded, a hole for ground-making and a hole for well sinking. In such cases, the hole should generally have a depth which is too long (e.g. ranging from 15 m to 20 m) for its diameter (e.g. about 5 m).
In deep excavating work, a construction machine is conventionally employed having a stretchable arm fixed to a boom wherein a clamshell bucket arrangement is coupled to a tip end of a top arm of the stretchable arm and is suspended vertically by the weight thereof. In this construction machine, the clamshell bucket is first hung to reach the bottom of the hole so as to bite into the sand and it closes after holding the sand. The clamshell bucket is successively pulled upward from the hole while it is closed and then it is opened over the ground or a truck bed, thereby transferring the held sand. If the operations are repeated, it is possible to excavate the earth deeper to form a deeper hole.
Such conventional clamshell bucket arrangement comprises a support shaft which forms a main frame thereof and is suspended from the top arm, two buckets supported by both sides of the lower end of the support shaft so as to be closable to the left and right, and teeth fixed to both sides of each bucket at the portion where these buckets confront each other. In such an arrangement, the sand can be held when the buckets supported by the support shaft are closed, and it can be dropped when the buckets are opened. The structure of this clamshell bucket is well known and is employed on construction machines when excavating soft ground.
In such a clamshell bucket arrangement, it is possible to easily excavate the earth by closing the buckets on soft earth which is not sticky such as a sandy soil, thereby increasing the working efficiency. However, in ground such as claylike (i.e. clayey) soil having fine granules and moisture content, the outer periphery of the clamshell bucket sticks to the clayey soil of the hole so that air does not enter the contact surfaces between the buckets and the clayey soil because of viscosity of the clayey soil, thereby causing the generation of a so-called sticky phenomenon where the buckets are stuck by the clayey soil. This phenomenon is caused by the peculiar shape of the clamshell bucket which is a circular part and can be turned as if circular arcs are traced. When the clamshell bucket turns relative to the ground, it bites into the soil like a circular arc so that the side surface of the clamshell bucket is brought into contact with or sticks to the soil along the circular arc which is generated by the tip end of the bucket. If the earth is clayey soil having fine granules, the sticky soil sticks to the outer periphery of the buckets, which prevents a gap, through which air enters, from being generated between the outer periphery of the buckets and the clayey soil so that the buckets become stuck to the clayey soil.
If the clamshell bucket becomes stuck to the clayey soil, it cannot be pulled upward, even if it is intended to be pulled up from the hole, so that a strong force is required to pull up the clamshell bucket. In such a state, the conventional deep excavator is likely to be damaged as it pulls out the clamshell bucket which is stuck to the clayey soil, or is likely to fall down. In such a case, the amount of soil which can be excavated by the bucket is reduced, thereby reducing the load applied to the clamshell bucket, which leads to ineffective working efficiency.
Accordingly, there has been proposed a construction machine to increase the capacity to pull up the weight and allowable load so as to cope with such a sticky phenomenon. However, if the excavator is designed to have such a large capacity, then such capacity is not needed in a normal excavating operation, and hence the machine is more complex and costly and yet such increased capacity is typically not frequently utilized.
In the conventional clamshell bucket having the aforementioned problems, it is often impossible to excavate the ground having sticky soil to form a deep hole. Accordingly, it is desired to develop a clamshell bucket capable of excavating such ground to form a deep hole without becoming stuck by the sticky soil.
In view of the aforementioned drawback of the conventional clamshell bucket arrangement, it is an object of the invention to provide a structure of a clamshell bucket and a hydraulic control circuit thereof capable of displacing the clamshell bucket relative to the hole in case of excavating clayey soil having high viscosity, thereby reducing the contact area between the clamshell bucket and the clayey soil and allowing air to enter into the contact surfaces therebetween so as to reduce the load applied to the clamshell bucket.
It is a first aspect of the invention to provide a structure of a clamshell bucket arrangement comprising a support shaft which is suspended substantially vertically, left and right buckets which are pivotally connected to both sides of a lower end of the support shaft, a pair of hydraulic cylinders which are interposed between the support shaft and the left and right buckets, wherein the left and right buckets are closed to hold soil when pistons of the hydraulic cylinders are extended and are opened to discharge the soil when the pistons of the hydraulic cylinders are contracted, and wherein the piston of one hydraulic cylinder has a stroke reserve so as to be further extendible so that the closed left and right buckets can be displaced to one side thereof relative to the vertical axes thereof.
It is a second aspect of the invention to provide a hydraulic control circuit of a structure of a clamshell bucket arrangement comprising a support shaft which is suspended substantially vertically, left and right buckets which are pivotally connected to both sides of the lower end of the support shaft, a pair of hydraulic cylinders which are interposed between the support shaft and the left and right buckets for opening and closing the left and right buckets when pistons of the hydraulic cylinders are contracted and extended, wherein the piston of one of the hydraulic cylinders has a stroke reserve so as to be further extendible in the longitudinal direction thereof even if the left and right buckets are closed, wherein the left and right buckets are closed to hold soil when the hydraulic cylinders are extended and are opened to discharge the soil when the pistons of the hydraulic cylinders are contracted, and wherein the hydraulic control circuit is characterized in that pressure chambers of the hydraulic cylinders are commonly connected to an oil supply passage through oil supply paths while discharge chambers of the hydraulic cylinders are commonly connected to an oil return passage through oil return paths, and a directional control valve is associated with the oil supply path of one hydraulic cylinder for disconnecting this hydraulic cylinder from the oil supply passage when hydraulic pressure of the oil in the oil passage is increased.
According to the present invention, the buckets are pivotally connected to both sides of the lower end of the support shaft and the hydraulic cylinders are interposed between the support shaft and each of the buckets at the back thereof. With the arrangement of this invention, it is possible to hold the soil by closing the buckets and to transfer the held soil to another place such as the bed of a truck, etc.
The hydraulic cylinders employed by the invention have different amounts of extension, namely, the amounts of extension of their pistons which are pulled out from them (hereinafter referred to as stroke length), and their stroke lengths are set in a manner such that when both pistons of the hydraulic cylinders are contracted at their maximum, both buckets are fully opened. Accordingly, when pistons of the hydraulic cylinders are extended so as to close both buckets, the piston of one cylinder is extended the maximum, but the piston of the other cylinder can be extended further (hereinafter called the piston of the cylinder having a stroke reserve). The stroke reserve corresponds to the difference of the stroke lengths of both pistons of the hydraulic cylinders, described hereinafter. As a result, when the buckets are stuck to the sticky soil such as clayey soil, the other piston is slightly extended relative to its cylinder while the one piston is contracted so that the pair of buckets can be sidewardly turned in one direction while the teeth of the buckets mesh one another to maintain the buckets closed. Consequently, both buckets are displaced from their central axes while they are kept closed so that the center of the hole and the center of the buckets are not aligned with each other, thereby reducing the areas of the buckets which contact the soil sticking to the buckets (hereinafter called the contact area of the buckets).
At this state, if the clamshell bucket is pulled up, air enters the gap defined between the buckets and the soil so as to easily pull the buckets apart from the soil. Further, since the contact area of the buckets is reduced, it is possible to reduce the force to pull the buckets from the hole compared with the case where the entire surfaces of the buckets are stuck by the soil. When the buckets are pulled out from the hole, both buckets can be freely turned due to their weight so that the oil under pressure can freely flow between both hydraulic cylinders. As a result, the central axes of the buckets can be realigned with their vertical lines .